1. Field of the Invention
The present invention relates to a pipeline suitable for use in transporting carbon dioxide containing materials such as crude oil, natural gas or the like, the pipeline having resistance to corrosion caused by the carbon dioxide. The present invention also relates to a steel pipe suitable for use in the pipeline, as well as to a circumferential welding method for constructing the pipeline with such steel pipe.
2. Description of the Related Art
It is a current measure to directly transport crude oil or natural gas through a pipeline from a submarine well or field to shore processing equipment where gas separation is conducted, rather than conducting gas separation in offshore equipment.
Since such crude oil or natural gas extracted from the submarine field contains carbon dioxide, the pipeline for transporting the oil or gas to the shore processing equipment tends to be corroded by carbon dioxide. Hitherto, therefore, an inhibitor which is mainly of the amine type, has been added to the material to be transported in order to prevent corrosion of the pipeline. Such an inhibitor, however, tends not to work satisfactorily at some local portions of the pipeline. In addition, no system has been established for dealing with troubles which are often caused in the system for injecting the inhibitor.
It is possible to suppress corrosion by using a corrosion resistant material as the material of the steel pipe of the pipeline. Such corrosion resistant metals, e.g., stainless steels or alloy steels, are prohibitively expensive, thereby not permitting use of such materials in long distance pipelines constructed by connecting large quantities of pipe pieces.
In general, resistance to corrosion caused by carbon dioxide can be improved by addition of chromium (Cr). Steel pipes for pipelines enriched in Cr are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 54-124817, 56-93856 and 55-158253. These disclosures, however, relate only to the strength of the base metals of the steel pipes. No successful approach has been made heretofore to improve resistance to carbon dioxide corrosion in due consideration of the weld metal used for connecting such steel pipes. Troublesome problems of local corrosion at circumferential weld portions of steel pipeline also remain unsolved.